Fungus Gnats

(Family Mycetophilidae.)
The flies which belong to this group are known as the fungus gnats, from the fact that many of them breed in fungi.

These insects are so delicate in structure that they are difficult to collect and study and are not so well known as they should be, although nearly a thousand have been described.
More than a hundred species have been described from the United States. They are delicate and as a rule rather slender little flies. The wings are generally clear, but sometimes they are smoky or with large spots as in those which belong to the genera Platyura, Sciophila and Mycetophila.

The female abdomen is frequently distended and expanded toward the tip, as in Platyura pectoralis Coq. and Asindulum montanum Roeder. With those species whose larvae live in fungi or decaying wood or other vegetable matter, the larvae are usually slender, cylindrical maggots, more or less worm-like in appearance. Some of them somewhat resemble snails and construct delicate cocoons.

It was formerly supposed that with some of the species the larvae formed galls on leaves, as, for example, one species was supposed to belong to the genus Sciara which makes the beautiful crimson, eye-like spots often seen on the leaves of the silver maple, but the larvae in these spots are now thought to be Cecidomyian and it is doubted whether any true Mycetophilids ever make galls.

Some of the Sciaras also have the curious habit when in the larval state of traveling in great armies so close to each other as to almost form one mass. They have then been called worm-snakes. They travel in a solid column several deep over each other at the rate of about an inch a minute. In Europe they have from this habit been called the army-worm, but in the U.S.A. the term army-worm is applied to a caterpillar. One of our American species of this habit has been reared by Pratt and proves to be Sciara fraterna.

Some species live in the sap of trees, and injury to the bark of a maple or an elm causing the sap to flow in the spring frequently attracts these little midges, which will lay their eggs there and subsequently little maggots will be found. One species, known as Epidapus scabiei, is said by Hopkins to be the cause, or at least the transmitter, of the disease known as scab among potatoes. He also finds that the same insect will breed in healthy potatoes.

The use of scabby seed potatoes offers favorable conditions for the attack of these insects as these are attracted to the scabby spots under which they breed and are thus brought into contact with the growing tubers.

Another species feeds upon ripe apples. One of the Sciaras has been called the yellow fever fly in the southern United States, since it made its appearance in extraordinary numbers during a yellow fever epidemic. It has, however, no connection with the disease. Certain of these fungus gnats jump actively as well as fly. Sciara critics Coq., figured herewith, in its larval stage damages young growing wheat plants.